Using X-ray and optical photometry of UBVRI fluxes, energetic compact extragalactic sources were studied, such as BL Lacerta (Lac) and BL Lac objects-Markarian (Mrk) 421, Mrk 501, and B2 1308+32. BL Lac and BL Lac objects were found to have predominantly nonthermal spectrums. Photometric and spectroscopic properties of X Persei, an unusual Be star associated with an intrinsically weak X-ray source, were also studied. Both its X-ray and optical emission show complex variability on many different time scales

Mufson, S.

English

NASA Technical Reports Server

CORRELATED GROUND BASED OBSERVATIONS IN SUPPORT 
OF THE HEAO-1 X-RAY OBSERVATORY 
Stewart Mufson 
Indiana University 
In this program our intent was to explore the physics associated 
with cosmic X-ray sources. Our objective was to be achieved by com- 
bining X-ray fluxes obtained by the A-1 experiment aboard the HEAO-1 
observatory with ground based optical and near infrared photometry and 
optical spectroscopy. The objects whose properties we have chosen for 
our study fall into two categories : ( 1) energetic, compact extragalactic 
sources, and (2 )  compact, galactic X-ray emitters like X Persei. In our 
inyestigation of the program sources, we observed UBVRI fluxes, and 
at times optical spectra, during periods chosen to coincide with obser- 
vations made by the HEAO-1 satellite. In addition to simultaneous 
experiments, a significant effort was made to monitor the sources. Since 
HEAO-1 only observes objects near dawn and dusk, we utilized the 
remainder of our observing sessions to establish optical baselines against 
which longer term X-ray variability could be compared. 
The photometric data obtained daring the HEAO-1 mission repre- 
sent observations made on 53 nights, extending back to the date origin- 
ally planned for the I-IEAO-1 launch, April 1977. These data include 11 
simultaneous X-ray and optical experiments performed during the HEAO-1 
mission. In addition, we have obtained seven nights of high time reso- 
lution optical spectroscopy in connection with our program. These data 
include three simultaneous experiments. In many cases we are currently 
continuing our optical observations of the program objects. 
In Figure 1 we show our UBVRI photometric data for BL Lacerta, 
an example of one type of compact, extragalactic source we have studied. 
Optical emission lines are absent in this source, so our optical photome- 
t ry ,  like X-ray photometry, is directly sampling the central engine 
without complications introduced by intervening gas [ I ] .  An average 
straight line has been drawn through this compilation of our photometric 
data. Clearly BL Lac has a predominantly nonthermal spectrum. The 
triangles are a set of observations obtained on one night, a snapshot, 
in which the spectrum was thought to be like the average spectrum, and 
the x's are a snapshot of the spectrum when it was considerably different 
from i ts  average. As is usual in BL Lac objects, the spectral shape 
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varies quite considerably in the course of time. The variations in the 
intensity of optical emission with time scales of days suggest the central 
engine in this source is quite small. 
In Figures 2 and 3 we see two low energy BL Lac objecth, Mark- 
arian (Mrk) 4 2 1  and Mrk 501, which have been identified with X-ray 
sources. The mini-lacertids in both these objects are embedded in giant 
elliptical galaxies [2]. These optical spectra are again predominantly 
nont hermal . Mrk 42 1 shows especially large variations in optical activity. 
The notation in these figures is the same as that used in Figure 1, 
where triangles mark a snapshot of an average spectrum and the x's 
show an example of a peculiar spectrum. In his talk this morning, D r .  
Schwartz made a very interesting point. From May 1977 to May 1978, 
the continuous X-ray spectrum of Mrk 421 was seen to steepen quite 
considerably [ 3,4]. In Figure 2 ,  the x's represent the optical spectrum 
of Mrk 4 2 1  in May 1978 and the average line is representative of the 
optical spectrum in May 1977. From May 1977 to May 1978 the optical 
spectrum, like the X-ray spectrum, was seen to steepen considerably. 
This correlated X-ray and optical behavior is what one would expect 
from a synchrotron-Compton source [ 51 . 
The combined UBVRI photometry for another BL Lac object we have 
studied closely - B2 1308+32 - is shown in Figure 4. The intensity 
variations seenlin this source are enormous. Our observational record 
also shows the optical spectrum has often changed i ts  shape. A cosmo- 
logical interpretation of its z = 0.996 redshift [6 ]  implies B2 1308+32 
has a peak luminosity of ergslsec; this luininosity is near the maxi- 
mum observed in the most energetic quasars 171. On the first pass by 
HEAO-1, the A 1  experiment found only an upper .limit to the X-ray flux 
in the energy range 3-20 keV. But as can be seen in Figure 5 ,  the 
combination of our B band photometry with other published values shows 
that the source was very quiescent during this satellite observation, 
and so not likely to have been seen in the X-ray. This figure also 
shows that B2 1308+32 flared very brightly in 1974 and 1977, llared 
moderately in 1975 and 1978, and was probably quite weak in 1976 and 
1979. We suggest the source will again be brightly flaring in 1980, and 
so will be an excellent candidate for a combined X-ray and optical study. 
The characteristic form of an optical burst has been followed now 
through almost three cycles. In Figure 5, the optical emission can be 
seen to rise slowly to a maximum, and then abruptly fall off. During 
spring 1978, such an outb.urst was particularly well marked. In Figure 
6 we display our UBV photometry during this burst using flux values 
normalized with the frequency independent units of energy flux (ergs/ 
cm2-sec). The energy flux rises more slowly than it  falls at all colors. 
The radio emission at 15 GHz during this outburst ( H  . Aller and T . 
Balonek, private communications) also rises gradually to a maximum in 
June. But the radio emission falls off far more slowly than the optical. 
In Figure 7 ,  we show the URVRI energy flux measurements during one 
night of monitoring observations. Variations at all colors can easily be 
seen. However, variations at different colors do n.ot appear to be 
correlated. As seen in Figures 4 through 7 ,  our optical studies so far 
imply the existence of two engines in this source - one for the long 
term intensity -variations which are correlated from radio to optical wave- 
lengths, and one for the emission of the rapid, uncorrclated intensity 
fluctuations observed at all colors. 
In Figure 8 we give our photometric data for the X-ray quasar 
3C 273. Our data for December 22, 1978 (x's) - near both HEAO-1 and 
HEAO-2 observations - shows that 3C 273 was brighter than any time 
since 1965 [ 81 . E.ven on December 27 , at the time of HEAO-A 1 pointed 
observations, this source was significantly brighter than average. 
Finally, in the second phase of our program, we have been inves- 
tigating the photometric and spectroscopic properties of X Persei, an 
unusual Be star associated with an intrinsically weak X-ray source. 
Both i t s  X-ray and optical emission show complex variability on many 
different time scalcs. At X-ray wavelengths the flux is observed to 
vary with a period of 13.9 min 191 ; there is in addition some evidence 
for a period of 2 2 . 4  h r .  A period of 580 days i s  well observed in the 
radial velocity of the Balmer lines [ lo] .  X Persei i s  most often inter- 
preted as a BOe star with a neutron star  companion. 
On August 2 3 ,  1978 we monitored the Hx emission Line during a 
HEAO-A1 pointed observation. The spectroscopic observations were 
made at the Goethe Link O b ~ e ~ v a t o r y  of Indiana University using an 
I-SIT spectrograph. The spectral resolution of this instrument is 2 a ,  
and each observatj.on lasted 90 see. In Figure 9 we show the equivalent 
width of Ha versus time for the two dates August 23, 1978 and September 
25, 1978. Our optical spec.troscopy recorded a remarkable event during 
the sim.~~l.taneous experiment. Boetween 9: 28 and 9 : 31 U'P' the equivalent 
width dropped from 7.3 to 3.3 A. This was followed by a more extended 
event of 540 sec duration in which the equivalent width dropped from 
7.7  to 2.6 a .  Standard stars were observed and the event does not 
appear to be instrumental in origin. During this event., the central 
wavelength increased and the line widths became narrower. This suggests 
a weakening of an unresolved, blueshift.ed line component. In the past 
observations of similar events have been reported [ 11,121. The X-ray 
data from this experiment i s  not yet available. On September 25, 1978 
the  equivalent width was higher ( 1 0 . 2  8,) and showed only a hint of 
variability over a three hour period. 
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Correlated ground based observations in support of the HEAO-1 X-ray observatory

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